Answer:
Gas ghosting is an undesirable phenomenon that occurs after the second pass through the press. An image that is printed on the first side of the press sheet appears as a ghosted image on the opposite side of the press sheet.

What causes it? After the first side of the press sheet is printed, the ink will "set" but isn't completely dry. There will be some oxidation as the ink dries. When an image is printed first on one side of the sheet, the solvents contained in the inks that print the image will absorb into a portion of the press sheet directly under the image. Then when an image prints on the other side of the sheet, the same ink solvents must absorb into the sheet as well. In the printed areas of the sheet that are already saturated from the first side, the solvents cannot penetrate as deeply, nor dry as quickly, thus causing a "ghost" of the image from the first side of the press sheet.

Typically, high-alkaline papers will ghost more often.

It can be difficult to predict when gas ghosting will occur, so be sure to work with your printer and paper supplier to reduce the chance of this phenomenon.

Jim Brisendine, Technical Sales Manager of PBM Graphics, Inc. answered this question. Thanks, Jim! For more information about PBM Graphics, contact Tom Arnold at 1-800-849-8100 or visit their Web site at www.pbmgraphics.com.